1. Field of the Invention
The field of the present invention relates in general to semiconductor processing. More particularly, the field of the invention relates to a system and method for thermally processing a semiconductor substrate using a stable temperature heat source.
2. Background
Diffusion furnaces have been widely used for thermal processing of semiconductor device materials (such as semiconductor wafers or other semiconductor substrates). The furnaces typically have a large thermal mass that provides a relatively uniform and stable temperature for processing. However, in order to achieve uniform results, it is necessary for the conditions in the furnace to reach thermal equilibrium after a batch of wafers is inserted into the furnace. Therefore, the heating time for wafers in a diffusion furnace is relatively long, typically exceeding ten minutes.
As integrated circuit dimensions have decreased, shorter thermal processing steps for some processes, such as rapid thermal anneal, are desirable to reduce the lateral diffusion of dopants and the associated broadening of feature dimensions. Thermal process duration may also be limited to reduce forward diffusion so the semiconductor junction in the wafer does not shift. As a result, the longer processing times inherent in conventional diffusion furnaces have become undesirable for many processes. In addition, increasingly stringent requirements for process control and repeatability have made batch processing undesirable for many applications.
As an alternative to diffusion furnaces, single wafer rapid thermal processing (RTP) systems have been developed for rapidly heating and cooling wafers. Most RTP systems use high intensity lamps (usually tungsten-halogen lamps or arc lamps) to selectively heat a wafer within a cold wall clear quartz furnace. Since the lamps have very low thermal mass, the wafer can be heated rapidly. Rapid wafer cooling is also easily achieved since the heat source may be turned off instantly without requiring a slow temperature ramp down. Lamp heating of the wafer minimizes the thermal mass effects of the process chamber and allows rapid real time control over the wafer temperature. While single wafer RTP reactors provide enhanced process control, their throughput is substantially less than batch furnace systems.
FIG. 1 is a graph illustrating a desired heating profile for a wafer during rapid thermal processing in a lamp heated RTP system. In particular, the solid line in FIG. 1 is a plot of the temperature of the center of a wafer over the duration of a rapid thermal annealing process. As shown in FIG. 1, the wafer may be heated at a rapid rate as indicated at 102 in FIG. 1. Lamp radiation may be rapidly adjusted as a desired processing temperature is approached in order to achieve a constant processing temperature, as indicated at 104. At the end of the processing step, the lamp radiation may be quickly reduced to allow cooling as indicated at 106.
While RTP systems allow rapid heating and cooling, it is difficult to achieve repeatable, uniform wafer processing temperatures using RTP, particularly for larger wafers (200 mm and greater). The temperature uniformity is sensitive to the uniformity of the optical energy absorption as well as the radiative and convective heat losses of the wafer. Wafer temperature nonuniformities usually appear near wafer edges because radiative heat losses are greatest at the edges. During RTP the wafer edges may, at times, be several degrees (or even tens of degrees) cooler than the center of the wafer. At high temperatures, generally greater than eight hundred degrees Celsius (800.degree. C.), this nonuniformity may produce crystal slip lines on the wafer (particularly near the edge). To minimize the formation of slip lines, insulating rings are often placed around the perimeter of the wafer to shield the wafer from the cold chamber walls. Nonuniformity is also undesirable since it may lead to nonuniform material properties such as alloy content, grain size, and dopant concentration. These nonuniform material properties may degrade the circuitry and decrease yield even at low temperatures (generally less than 800.degree. C.). For instance, temperature uniformity is critical to the formation of titanium silicide by post deposition annealing. In fact, the uniformity of the sheet resistance of the resulting titanium silicide is regarded as a standard measure for evaluating temperature uniformity in RTP systems.
Temperature levels and uniformity must therefore be carefully monitored and controlled in RTP systems. Optical pyrometry is typically used due to its noninvasive nature and relatively fast measurement speed which are critical in controlling the rapid heating and cooling in RTP. However, accurate temperature measurement using optical pyrometry depends upon the accurate measurement of the intensity of radiation emitted from the wafer and upon the wafer's radiation emitting characteristics or emissivity. Emissivity is typically wafer dependent and depends on a range of parameters, including temperature, chamber reflectivity, the wafer material (including dopant concentration), surface roughness, and surface layers (including the type and thickness of sub-layers), and will change dynamically during processing as layers grow on the surface of the wafer. In addition, radiation from heat sources, particularly lamps, reflect off the wafer surface and interfere with optical pyrometry. This reflected radiation erroneously augments the measured intensity of radiation emitted from the wafer surface and results in inaccurate temperature measurement.
Increasingly complex systems have been developed for measuring emissivity and for compensating for reflected radiation. One approach uses two optical pyrometers--one for measuring the radiation from the lamps and one for measuring the radiation from the wafer. The strength of the characteristic AC ripple in radiation emanated from the lamp can be compared to the strength of the AC ripple reflected from the wafer to determine the wafer's reflectivity. This, in turn, can be used to essentially subtract out reflected radiation in order to isolate the emitted radiation from the wafer for determining temperature using Planck's equation. See, e.g., U.S. Pat. No. 5,166,080 to Schietinger et al. However, such systems may require complex circuitry to isolate the AC ripple and perform the calculations that effectively eliminate reflected radiation. Such systems also require an additional optical sensor and other components.
Another approach for measuring wafer temperature and compensating for the effects of emissivity uses an infrared laser source that directs coherent light into a beam splitter. From the beam splitter, the coherent light beam is split into numerous incident beams which travel to the wafer surface via optical fiber bundles. The optical fiber bundles also collect the reflected coherent light beams as well as radiated energy from the wafer. In low temperature applications, transmitted energy may be collected and measured as well. The collected light is then divided into separate components from which radiance, emissivity, and temperature may be calculated. See, e.g., U.S. Pat. No. 5,156,461 to Moslehi et al. It is a disadvantage of such systems that a laser and other complex components are required. Such systems, however, are advantageous because they may provide measurements of wafer temperature at multiple points along the wafer surface which may be useful for detecting and compensating for temperature nonuniformities.
In order to compensate for temperature nonuniformities, a heater with multiple independently controlled heating zones may be required. One approach is to use a multi-zone lamp system arranged in a plurality of concentric circles. The lamp energy may be adjusted to compensate for temperature differences detected using multi-point optical pyrometry. However, such systems often require complex and expensive lamp systems with separate temperature controls for each zone of lamps. For instance, U.S. Pat. No. 5,268,989 to Moslehi et al. discloses a multi-zone heater with sixty five tungsten-halogen lamps arranged into four heating zones. In addition, a light interference elimination system is disclosed which uses light pipes in seven dummy lamps to measure lamp radiation as well as five or more light pipes for measuring radiation across the surface of the wafer. The light interference elimination system uses the radiation of the dummy lamps to determine the fraction of total radiation from the wafer surface that is reflected from the lamps as opposed to emitted from the wafer surface. The emitted radiation can then be isolated and used to detect temperature across the wafer surface, which in turn can be used to control the lamp heating zones.
A widely used exemplary RTP system is the Heatpulse.TM. 8108 system from AG Associates shown in cross section in FIG. 2. According to published technical specifications, this system uses twenty eight tungsten halogen lamps in cross configuration with ten software controlled heat zones. In process specifications for this system, the uniformity of titanium silicide formation on a 200 mm wafer is reported to be 1.5% nonuniformity added to as-sputtered titanium wafer uniformity. A throughput of around twenty five (25) wafers per hour is reported for this process.
While multi-zone lamp systems have enhanced wafer temperature uniformity, their complexity has increased cost and maintenance requirements. In addition, other problems must be addressed in lamp heated RTP systems. For instance, many lamps use linear filaments which provide heat in linear segments and as a result are ineffective or inefficient at providing uniform heat to a round wafer even when multi-zone lamps are used. Furthermore, lamp systems tend to degrade with use which inhibits process repeatability and individual lamps may degrade at different rates which reduces uniformity. In addition, replacing degraded lamps increases cost and maintenance requirements.
In order to overcome the disadvantages of lamp heated RTP systems, a few systems have been proposed which use a resistively heated plate. Such heated plates provide a relatively large thermal mass with a stable temperature. FIG. 3 shows a side cross sectional view of a conventional heated plate rapid thermal processor. Referring to FIG. 3, a wafer may be placed on or near a heated plate 304 for thermal processing. The wafer enters the chamber through a port 306 formed in the chamber wall 308. The wafer is placed on support pins 310 which may be raised and lowered for loading and unloading the wafer. For processing, the wafer is lowered onto or close to the heated plate 304. The heated plate is heated by a resistive heater 312, and the wafer is rapidly heated by conduction, convection, and radiation from the heated plate. Since the heated plate is a constant and substantial source of heat, a reflective heat shield 314 may be necessary in order to protect the chamber walls 308. Temperature is monitored in the system of FIG. 3 using a thermocouple 316 disposed in the heater plate, as opposed to an optical pyrometer which may be affected by emissivity variations. However, it is the temperature of the heated plate 304 that is directly measured by the thermocouple and not the temperature of the wafer.
While the heated plate 304 provides a stable, repeatable heating source with a large thermal mass, similar to a diffusion furnace, the chamber walls 308 be cooled. This allows a wafer to be rapidly heated by lowering the wafer onto the heated plate for a short period of time and rapidly cooled by removing the wafer from the plate. In addition, a radiation absorbing material may be used to coat the top surface of the chamber to enhance cooling as the wafer is raised by the pins after heating. See U.S. Pat. Nos. 5,060,354 and 5,252,807 to Chizinsky.
While heated plate rapid thermal processors provide a stable temperature on the heated plate that may be measured using a thermocouple, problems may be encountered with wafer temperature nonuniformities. Wafers may be heated by placing them near the heated plate rather than on the plate. In such systems, the edges of the wafer may have large heat losses which lead to nonuniformities as in lamp heated RTP systems. Even when a wafer is placed in contact with a heated plate, there may be nonuniformities. The heated plate itself may have large edge losses, because: 1) the corners and edges of the plate may radiate across a wider range of angles into the chamber; 2) vertical chimney effects may cause larger convective heat losses at the edges of the heated plate; and 3) the edges of the heated plate may be close to cold chamber walls. These edge losses on the plate may, in turn, impose temperature nonuniformities upon a wafer placed on the plate.
In addition, heat loss and temperature uniformity across the wafer surface varies with temperature and pressure. Conductive heat transfer between two objects (such as the wafer and the cold chamber wall) is proportional to the temperature difference between the objects and radiative heat transfer is proportional to the difference of the temperatures raised to the fourth power (T.sub.1.sup.4 -T.sub.2.sup.4). Thus, the difference between temperatures across the wafer surface will increase at higher processing temperatures. In addition, the pressure in the chamber may affect the wafer temperature profile since heat transfer at low pressures is predominantly carried out by radiation, while heat transfer at higher pressures involves a combination of radiation, conduction and convection.
As with lamp heated RTP systems, a variety of techniques may be used to enhance wafer temperature uniformity. For instance, the reactor of FIG. 3 includes a wall 317 extending upward from the perimeter of the heated plate. The wall 317 is intended to help maintain the uniformity of the temperature across the diameter of the wafer, as the wafer is displaced on the pins, away from the heated plate. However, it is believed that the cold chamber walls, which are close to and directly exposed to the upstanding wall and portions of the heated plate, will induce temperature and process nonuniformities. In addition, the effect of the wall will vary across temperature and pressure ranges.
Conventional heated plate processing systems also tend to be energy inefficient. The heated plate is maintained at a high temperature with constant conductive, convective and radiative losses to the cold chamber walls. While conductive and convective losses may be reduced at lower pressures, this inhibits the heat transfer to the wafer. At low pressures where heating is primarily radiative, the wafer may be significantly cooler than the heated plate particularly when proximity heating is used. This makes the wafer temperature difficult to control. Further, at low pressures where radiation is the primary mechanism for heat transfer, the variance in wafer temperature uniformity across temperature ranges may be greater because heat transfer by radiation is proportional to the difference between surface temperatures raised to the fourth power (T.sub.1.sup.4 -T.sub.2.sup.4). Thus, decreasing pressure to increase energy efficiency may make the wafer temperature and uniformity more difficult to control.
Another disadvantage associated with conventional heated plate processors is that their large thermal mass prevents the rate of heating from being rapidly adjusted to achieve desired temperature profiles, such as the rapid thermal anneal profile shown in FIG. 1. When a wafer is placed near a constant temperature heat source, such as a heated plate with a large thermal mass, it has an asymptotic temperature profile over time as shown in FIG. 4. The wafer initially heats rapidly as shown by the portion of the curve indicated at 404. As the wafer temperature approaches the temperature of the plate, the rate of heating slows and the temperature of the wafer approaches the temperature of the heated plate asymptotically as shown by the portion of the curve indicated at 406. Since the large thermal mass prevents the temperature of the heated plate from being rapidly adjusted, the desired temperature profile of FIG. 1 will not be achieved.
Additional problems may also be encountered in conventional heated plate processors. In particular, a graphite heater may be desired due to its advantageous heating properties; however, graphite heaters are often fragile and easily damaged by shear strain. Thus, a graphite heater may be damaged when it is clamped or mounted to a support or electrode, and it is often difficult to provide a reliable electrical connection between a graphite heater and a power source. In addition, if a heater is mounted with a vertical support as indicated at 318 in FIG. 3, it may expand vertically during heating. This necessitates a clearance distance between the resistive heater and the heated block to allow for different levels of expansion at different temperatures. However, for efficient heating it is preferred that the resistive heater be closely spaced to the heated block.
As a result of the problems associated with conventional heated plate rapid thermal processors, they have not been adopted in the industry as a viable alternative to lamp heated RTP systems. A 1993 survey of RTP equipment covering twenty two different vendors' products indicates that, at the time of the survey, only one non-lamp system was available. See Roozeboom, "Manufacturing Equipment Issues in Rapid Thermal Processing," Rapid Thermal Processing at 349-423 (Academic Press 1993). The only non-lamp system listed uses a resistively heated bell jar with two temperature zones and is not a heated plate reactor. See U.S. Pat. No. 4,857,689 to Lee. Currently, the RTP market is dominated by lamp based systems and despite the many problems associated with such systems, they have been widely accepted over proposed heated plate approaches. Despite the potential that heated plate approaches offer for a stable and repeatable heat source, it is believed that problems with energy efficiency, uniformity, temperature and heating rate control, and the deployment of fragile, noncontaminating resistive heaters have made such systems unacceptable in the marketplace.
What is needed is a system and method for rapid thermal processing with a stable and repeatable heating source that provides a high level of uniformity across a wide range of temperatures. Preferably, the heating source would be maintained at a high temperature without necessitating rapid heating and cooling of the heating source. In addition, such a system would preferably be energy efficient while providing accurate wafer temperature control that is substantially independent of variances in wafer emissivity and would allow a cold walled chamber to be used. Preferably such a system would also provide substantially improved throughput over conventional single wafer RTP systems while maintaining a high level of process control and wafer temperature uniformity. Such a system would also preferably provide a compact heating source that is not significantly larger than the wafers being heated.
What is also needed is a system and method for thermal processing of a wafer using a heating source with a relatively large, stable thermal mass while allowing the rate of heating to be rapidly adjusted to achieve desired temperature profiles. Preferably such a system would allow a wafer to be heated at a rapid rate until a desired temperature is achieved and then allow the rate of heating to be quickly adjusted to maintain the temperature at a relatively constant level. In addition, such a system would preferably allow thermal processing of wafers with a temperature profile, uniformity and throughput competitive with conventional lamp RTP systems.
What is also needed is an improved system and method for deploying a fragile, resistive heater. Preferably such a system would allow a graphite heater to be mounted to a power source with an improved electrical connection and with substantially reduced potential for damage due to shear strain. In addition, such a system would preferably allow a graphite heater to be mounted closely to a heated block without significant vertical expansion across a wide range of temperatures.
Preferably, each of the above features would be combined in a single compact, cost-effective RTP system and method.